Azobenzene-based optoelectronic transistors for neurohybrid building blocks

Exploiting the light–matter interplay to realize advanced light responsive multimodal platforms is an emerging strategy to engineer bioinspired systems such as optoelectronic synaptic devices. However, existing neuroinspired optoelectronic devices rely on complex processing of hybrid materials which often do not exhibit the required features for biological interfacing such as biocompatibility and low Young’s modulus. Recently, organic photoelectrochemical transistors (OPECTs) have paved the way towards multimodal devices that can better couple to biological systems benefiting from the characteristics of conjugated polymers. Neurohybrid OPECTs can be designed to optimally interface neuronal systems while resembling typical plasticity-driven processes to create more sophisticated integrated architectures between neuron and neuromorphic ends. Here, an innovative photo-switchable PEDOT:PSS was synthesized and successfully integrated into an OPECT. The OPECT device uses an azobenzene-based organic neuro-hybrid building block to mimic the retina’s structure exhibiting the capability to emulate visual pathways. Moreover, dually operating the device with opto- and electrical functions, a light-dependent conditioning and extinction processes were achieved faithful mimicking synaptic neural functions such as short- and long-term plasticity.

A preliminary analysis was performed, acquiring a survey spectrum of the bare PEDOT:PSS showing all characteristic peaks of the atoms present in the polymer structure: O 1s (24,4%), C 1s (60,8%), and S 2p (4,80%).These results are in agreement with those reported in the literature 5 .To evaluate the amount of entrapped PSS -during electrochemical polymerization, two different oxidation states of sulfur atoms (S 2p) were evaluated through the acquisition of XPS spectra in high-resolution mode (core level spectra).A doublet peak at 164.0 eV and 165.1 eV corresponding to the sulfur atom peaks contained in the thiophene ring of the PEDOT backbone was observed 6 .Moreover, an additional doublet peak was noted at 168.0 eV and 169.0 eV, which corresponds to the sulfur atoms present in the PSS moieties entrapped in the polymer film 6 .By considering the peak areas, the PEDOT/PSS ratio was calculated to be 1:2.25, as also previously shown 7 .
electron-deficient nitrogen (-N=N=N) and to the two partially negatively charged nitrogen atoms (-N=N=N) of the -N3 group.All data agree with those reported in the literature 8 .Both peaks decrease dramatically in the core level spectra of azo-tz-PEDOT:PSS, confirming the disappearance of the -N3 group after the functionalization reaction.
Moreover, a broadening of the peak at 400.6 eV was perfectly fitted by the tabulated peaks of the triazole ring (400.7 eV and 401.7 eV), as reported in the literature 8 .

Supplementary Figure 7 .
XPS analysis of bare PEDOT:PSS.XPS a) survey spectrum of PEDOT:PSS and b) S 2p core level spectrum of PEDOT:PSS.The surface elemental composition of the bare PEDOT:PSS electrodeposited film and the amount of entrapped PSS during electrochemical polymerization were investigated through XPS analysis.

Figure 9 .
Morphology characterization through acquisition of AFM and SEM.AFM images of a) PEDOT:PSS, b) N3-PEDOT:PSS and c) azo-tz-PEDOT:PSS films.SEM images of d) PEDOT:PSS, e) N3-PEDOT:PSS and f) azo-tz-PEDOT:PSS films.The morphologies of electrodeposited PEDOT:PSS, N3-PEDOT:PSS and azo-tz-PEDOT:PSS films were characterized through atomic force microscopy (AFM) and scanning electron microscopy (SEM).The AFM analysis of bare PEDOT:PSS and N3-PEDOT:PSS shows a globular surface with PSS (dark regions) and PEDOT domains (bright regions) randomly distributed in the polymer matrix, in good agreement with the literature data 2 .The AFM image of azo-tz-PEDOT:PSS does not show significant differences in the topography of the film.Here, PEDOT:PSS, N3-PEDOT:PSS and azo-tz-PEDOT:PSS have a mean RMS roughness of 15.7 ± 2.0 nm, 10.7 ± 2.0 nm and 10.5 ± 3.1 nm, respectively (N = 3).Additionally, SEM images of the azo-tz-PEDOT:PSS show a smooth morphology (N = 3) similar to the N3-PEDOT:PSS precursor and PEDOT:PSS, suggesting that the 'click' reaction only introduces a limited smoothening of the surface.Supplementary Figure 13.OPECT fabrication process.A) N3-PEDOT:PSS electrodeposition (blue square), B) isolation of ITO square electrodes with Kapton tape, C) functionalization of the polymer with azo-alkyne to give azotz-PEDOT:PSS (violet square), D) coverage of the azo-tz-PEDOT:PSS film with a rectangular PDMS mask (gray) and O2 plasma activation, E) spin coating of PEDOT:PSS solution, F) isolation of the desired part of sPEDOT:PSS corresponding to the OPECT channel G) O2 plasma etching to afford the desired dimension and geometry of sPEDOT:PSS channel.The fabrication of the OPECT bearing the azo-tz-PEDOT:PSS as a planar gate involved 7 steps.Patterned ITO substrates were previously cleaned in an ultrasonic bath for 10 minutes in each of the following solvents: Alconox® detergent solution, DI water, acetone, and 2-propanol.Then, N3-PEDOT:PSS was electrodeposited on the upper part of the ITO-coated glass and functionalized following the previously described click chemistry reaction procedure (Methods), insulating first the bottom part of ITO with Kapton tape.The azo-tz-PEDOT:PSS gate was then covered with a PDMS mask, and the bottom part of the ITO was activated through O2 plasma treatment (20 W, 2 minutes) to allow spin coating of the PEDOT:PSS (sPEDOT:PSS) channel.The sPEDOT:PSS layer was then O2 plasma etched (100 W, 15 minutes) through a PDMS hard mask to obtain the desired channel geometry and dimension.